The dental specialty orthodontics is concerned with the correction of alignment and positional abnormalities of the teeth. It is not uncommon for patients receiving such treatment to require a regimen which continues over many months and involves the use of various appliances affixed within the mouth to achieve repositioning of displaced teeth. The repositioning is accomplished, generally, by attachment of an orthodontic appliance to one or several of the teeth in order to provide forces on the affected teeth which accomplish the desired repositioning.
Often it is necessary for the orthodontic clinician to reposition a patient's maxillary and mandibular first permanent molars by de-rotating the molars or expanding the distance between the molars. This procedure, in the case of maxillary first permanent molars, is often accomplished during the course of generally expanding the palate to properly position the molars and reduce crowding of the upper arch interior teeth as well as to adjust the occlusion and bite.
To achieve this expansion of the molars and the palatal arch it has been common practice to utilize various types of arch bars or jack screws which are positioned between the maxillary molars to achieve rotation of the molars and to accomplish the desired expansion of the palate.
Typically these types of devices require a number of weeks or months of action on the teeth to accomplish the desired goal. In the case of stainless steel type arch wires the orthodontic appliance operates by simple mechanical pressure against the lingual side of the molars. The stainless steel appliance's ability to expand the palate is limited to the steel's capacity to withstand compression before reaching its yield point. This limitation of stainless steel requires periodic return visits to the orthodontist so the appliance can be reformed.
This periodic reforming of the appliance is necessary to restore the desired moment-of-force applied by the device and to realign the direction of force application with respect to the teeth. This is accomplished by modifying the shape of the appliance. This shape modification is necessary as often the stainless steel appliance must be initially bent into incorrect alignments to permit affixation to the mispositioned teeth.
Usually, the tooth repositioning process with stainless steel involves the incorporation of U-shaped bends into the arch which are slowly expanded over weeks to maintain force on the teeth and to increase the distance of separation between the molars. Such treatment requires multiple appointments with the orthodontic clinician and removal of the arch wire from the patient followed by reshaping of the device and reinstallation. This procedure can be time consuming and require substantial patient chair time.
In the case of jack-screw type palatal expansion devices, the expansion screw is affixed to the molars and over a period of several weeks the teeth are expanded by having the patient lengthen the screw by quarter turn increments on a daily basis. This type of device requires patient involvement in the treatment. Patient involvement may be improperly attended to or not conducted at all. In addition, patient manipulation of the device can result in damage to the appliance or injury to the patient. The vagaries of patent involvement are best avoided for most efficient results.
More importantly, such jack-screw type devices operate by the initial application of forces in the range of hundreds of grams. Forces on the order of 500 to 1300 grams are initially applied to the teeth when the jack screw is manipulated in the clinician's office. This initial high force application to the teeth is then followed by rapid movement of the teeth and a nearly immediate decay of the force to a pressure of zero. Such intermittent high force is considered, by orthodontic researchers, to be almost detrimental to the overall result and is suspected of resulting in retrograde bone destruction.
Recently, the introduction of improved metal alloys to the art of orthodontic appliance manufacture has resulted in the ability to provide palatal arch expansion devices and other orthodontic appliances which can apply force to the teeth over a much greater range of distances. Previously, with stainless steel type devices, the range of motion of the orthodontic appliance was limited by the distance over which stainless steel could be compressed and still maintain its tendency to spring back to its original shape. As the capacity of stainless steel to be bent or compressed without yielding is limited, multiple patient visits were required to accomplish the desired movement of teeth over the full distance to achieve proper positioning.
This limitation of stainless steel has been overcome by the use of metal alloys which present a "shape memory" which allows the orthodontic appliance to be bent and twisted to a much greater extent than a stainless steel device without resulting in a permanent reconfiguration of the device.
One such type of alloy is the nickel-titanium (Ni--Ti) alloy combinations, generally known as Nitinol, and is sold commercially under the trademark "Tinel" by Raychem Corp. of Menlo Park, Calif. Nitinol alloy is a known near-stoichiometric alloy of nickel and titanium. The alloy may also include cobalt substituted for nickel on an atom-for-atom basis so that the composition is Ni--Ti; Co: 0.935, 0.065.
Nickel-titanium alloy metals offer unique physical characteristics. Of particular interest in the field of orthodontics is the property of conformation or shape memory of the metal. That is, the alloy presents temperature dependent flexibility properties and strength properties which can be utilized to particular advantage in orthodontic appliances. Such nickel-titanium alloys present two types of physical states, the transition between which is temperature activated. The particular temperature at which the metal shifts from one state to another is dependent upon the contents of the alloy. At temperatures below the transition temperature the metal is in its "martensitic" state. In this state the metal is comparatively soft and may be deformed and twisted with relative ease. This property of the alloy is of particular utility in orthodontics for the metal, when in its martensitic state, may be twisted and greatly distorted without surpassing the metals yield point which would result in permanent conformation changes. Thus, this property permits the orthodontic clinician to bend and twist the orthodontic appliance during the application session without damaging the appliance.
As utilized in orthodontic appliances these metal alloys provide corrective force by taking advantage of the body temperature of the patient to provide the transition temperature of the particular metal alloy composition. When the metal is at a temperature above the transition temperature it is said to be in its "austenitic" state within which the metal seeks to return to a predetermined shape. It is this property of reverting to an original conformation at temperatures above the transition temperature which provides the forces useful in orthodontic appliances. The properties of Nitinol permit the application of low forces to orthodontic applications and forces which are substantially consistent over the range of motion as the appliance moves back into its original or "memory" shape.
However, nickel-titanium and similar alloys present the limitation in that welding the metal is not possible. The heat of welding will result in modification or destruction of the "memory" conformation. Therefore, to accomplish attachment of the Nitinol orthodontic device to the teeth, it is necessary to use a crimp tube adapter to join a second workable metal to the Nitinol. An example of such a prior art method of securing a Nitinol arch is illustrated in FIGS. 10 and 11.
Such tube crimp connectors inhibit the joining of more than one Ni--Ti wire to another Ni--Ti wire or to a sheath insert or end piece. If the clinician desires to use multiple Ni--Ti wires in a treatment, several such crimp tubes must be soldered together, either one atop the other or some other arrangement, to permit the use of multiple Ni--Ti segments in the appliance. This results in an expensive and cumbersome device and one which presents increased opportunity for breakage with each soldered connection.